Fiber optic couplers created for the purpose of splitting (or coupling) the optical power to more than one optical fiber have been widely used. Depending on a desired operation or the coupling function (i.e., coupling ratio, wavelength band of interest, polarization of the transmitted and coupled mode), wide band couplers, wave length independent couplers, single mode couplers, polarization maintaining (PM) couplers, PM hybrid couplers and other couplers have been used. When incorporated in optical systems, the couplers frequently operate at varying temperatures and stress.
Optical systems such as optical communication systems, optical gyroscopes, interferometric sensors require propagation and coupling of light with predetermined polarization mode. Different PM couplers have been described in the literature. FIGS. 1 and FIG. 1A show a polarization maintaining coupler that is similar to a coupler described in U.S. Pat. No. 4,906,068 and other publications. The coupler is embedded in a channel 16 of a rectangular substrate 8, but a circular substrate can also be used. Fibers 10 are embedded in an adhesive 18 that fills the space defined by channel 16 and a cover 20. In seeking to achieve undisturbed propagation of a polarized mode in PM fibers 10 it has been sought to position the fibers on the channel axis in order to create a relatively uniform cross-section of adhesive 18 in which the coated optical fibers 10 (or fibers with exposed cladding 12) are centrally embedded. Cover 20 located on top of channel 16 protects coupling region 14 and, in the ideal situation, helps to maintain axial symmetry of the forces on the fibers that are completely surrounded by the adhesive.
There are several problems with manufacturing and operation of this type of a coupler due to the external forces acting on the optical fibers and thereby disturbing the maintenance of polarization (i.e., creating "cross-talk" or energy transfer between the fast and slow eigenmodes of the polarization maintaining fibers). It is difficult to place the fibers symmetrically within the channel, so that uncontrollable asymmetries occur. To maintain a zero net external force on the fibers, it would be desirable to match the thermal coefficients of expansion of the fibers and the adhesive. However, this is a very difficult if not impossible to achieve, and thus, due to differences in thermal expansion and contraction, thermal changes produce polarization-disturbing forces on the fibers. Another major problem arises upon curing adhesive 18 that shrinks and, if the adhesive non-uniformly surrounds the fiber cover it exerts non-uniform force on the fiber. It is also believed that this method of fabrication enhances any small imperfection in the fibers created in the manufacturing process; the imperfections enhance the cross-talk. In some couplers, the epoxy region beneath cover 20 extends from the polymer covered (buffered) portion of the fibers to the bare cladding exposed portion of the fibers. This type of bonding further disturbs the polarized mode propagating in the fiber.
Furthermore, when the PM coupler is used in a cold environment the adhesive contracts further applying additional stress on the fiber. The stress causes asymmetrical forces which induce cross-talk between the polarized modes in the fiber.
In general, the couplers described by the prior art exhibit a significantly higher cross-talk than 1%, particularly when the coupler is used at low temperatures.
There is a need to create polarization maintaining couplers that can achieve a cross-talk substantially less than 1% between the desired and undesired polarization mode at low temperatures.
Even in contexts not dependent upon maintenance of polarization, there is also a need to have a bonding process that results in a more dependable and uniform mounting of couplers in protective packages.